To improve flame retardancy of thermoplastic resin composition is a major target to the research and development of the resin for a long time. The flame retardancy is measured by the UL-94 test method of Underwriters Laboratory. In this flame retardancy test, if the specimen is flamed out within 10 seconds after flaming and if the total flame out time is within 50 seconds when five specimens are flamed twice, the resin composition can obtain V-0 degree of flame retardancy.
A blend of a polycarbonate resin and a styrene-containing copolymer is a resin composition which has improved processability maintaining the good notched impact strength. The polycarbonate resin composition should further have good flame retardancy as well as high mechanical strength because the resin composition are applied to electric or electronic goods, automobile parts, office supplies, etc.
To provide the polycarbonate resin with good flame retardancy, a halogen-containing flame retardant and/or an antimony-containing compound were used. However, the disadvantages could be observed that the halogen-containing compound results in the corrosion of the mold itself by the hydrogen halide gases released during the molding process and is fatally harmful due to the toxic gases liberated in case of fire. Especially, since the polybromodiphenyl ether, mainly used for a halogen-containing flame retardant, can produce toxic gases such as dioxin or duran during combustion, flame retardants which are prepared without a halogen-containing compound have become a major concern in this field. Accordingly, It is a trend to prepare a resin composition using a halogen-free phosphorous compound.
Phosphorous compounds are a representative halogen-free flame retardant. The phosphorous compounds include monophosphate such as triphenylphosphate and tricresilphosphate, and oligomeric phosphate such as resorcinol bis(diphenylphosphate), hydroquinone bis(diphenylphosphate) and bisphenol-A bis(diphnylphosphate). However, as the halogen-free flame retardant shows a poorer flame retardancy than the halogen-containing flame retardant, an excess amount of halogen-free flame retardant should be used to obtain the same degree of flame retardancy. If an excess amount of the flame retardant is used, the impact strength and heat resistance of the resin composition can become severely deteriorated.
Japanese Patent Laid-open No. 2000-154277 discloses a thermoplastic resin composition using phosphoric acid amide as a flame retardant, which improves processability of the resin and hydrolysis resistance of the flame retardant. However, particular phosphoric acid amides cause poor hydrolysis resistance of the flame retardant resin composition. Further, in certain phosphorous acid amides, the flame retardancy and impact strength are severely decreased and the flame retardant becomes volatile to cover the surface of the molded article, so called, to occur juicing phenomenon.
Japanese Patent Laid-open No. 6-100,785 discloses a flame retardant resin composition which comprises a thermoplastic resin, a phosphate compound, and a silicon resin or a polyphosphazene to prevent from dripping of the resin during combustion. However, the Japanese patent application does not disclose that the flame retardancy of the resin composition had been improved.
EP 0 728 811 A2 discloses a thermoplastic resin composition comprising an aromatic polycarbonate, a graft copolymer, a copolymer and a phosphazene. The European patent teaches that no dripping occurs during combustion by using a phosphazene as a flame retardant even though an additional anti-dripping agent is not employed, and that the resin composition has excellent heat resistance and impact strength. However, the resin composition of the European patent has a poor processability due to the poor flowability by using the phosphazene, and causes black stripes and/or black lines on the surface of the molded article due to degradation of the resin or flame retardants when an excess stress is applied to the article during molding. In the European patent using phosphazene as a flame retardant, mechanical strength such as flexural strength and flexural modulus becomes deteriorated, and more flame retardants should be used to maintain a certain degree of flame retardancy.
WO 00/09518 and WO 99/19383 disclose methods of preparing a cross-linked linear or cyclic phenoxy phosphazene and a thermoplastic resin composition using such phosphazene compounds. According to the patent applications, the cross-linked phenoxy phosphazene does not deteriorate the mechanical properties of the resin composition when applied, because the phosphazene has a high melting point and lower volatility. However, the resin composition of the patent applications has a poor processability due to the poor flowability by using the phosphazene, and causes black stripes and/or black lines on the surface of the molded article due to degradation of the resin or flame retardants when an excess stress is applied to the article during molding. In the patent applications using phosphazene as a flame retardant, mechanical strength such as flexural strength and flexural modulus becomes deteriorated.
The present inventors have developed a flame retardant thermoplastic resin composition that comprises a polycarbonate resin, a rubber modified vinyl-grafted copolymer, a vinyl copolymer, a phosphorous compound mixture of a cyclic phosphazene oligomer compound and a phosphate ester morpholide compound as a flame retardant and a fluorinated polyolefin resin, which has a good balance of physical properties such as impact strength, heat resistance, heat stability, processability and appearance.